[javascript mode] Make tokenizing of async more context-sensitive

Closes codemirror#4774

Author: marijnh

mode/javascript/javascript.js

@@ -41,7 +41,7 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       "true": atom, "false": atom, "null": atom, "undefined": atom, "NaN": atom, "Infinity": atom,
       "this": kw("this"), "class": kw("class"), "super": kw("atom"),
       "yield": C, "export": kw("export"), "import": kw("import"), "extends": C,
-      "await": C, "async": kw("async")
+      "await": C
     };
 
     // Extend the 'normal' keywords with the TypeScript language extensions
@@ -147,9 +147,16 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       return ret("operator", "operator", stream.current());
     } else if (wordRE.test(ch)) {
       stream.eatWhile(wordRE);
-      var word = stream.current(), known = keywords.propertyIsEnumerable(word) && keywords[word];
-      return (known && state.lastType != ".") ? ret(known.type, known.style, word) :
-                     ret("variable", "variable", word);
+      var word = stream.current()
+      if (state.lastType != ".") {
+        if (keywords.propertyIsEnumerable(word)) {
+          var kw = keywords[word]
+          return ret(kw.type, kw.style, word)
+        }
+        if (word == "async" && stream.match(/^\s*[\(\w]/, false))
+          return ret("async", "keyword", word)
+      }
+      return ret("variable", "variable", word)
     }
   }
 

mode/javascript/test.js

@@ -226,6 +226,10 @@
      "  [property method]: [string 'GET']",
      "});");
 
+  MT("async_variable",
+     "[keyword const] [def async] [operator =] {[property a]: [number 1]};",
+     "[keyword const] [def foo] [operator =] [string-2 `bar ${][variable async].[property a][string-2 }`];")
+
   MT("indent_switch",
      "[keyword switch] ([variable x]) {",
      "  [keyword default]:",